I say Nick Fury. It would be a great way to make Thanos a major threat, to completely take away the reason the Avengers gathered together in the first place. I also feel that Thanos should be a multi-film threat, like last from Phase 2 into Phase 3.

Thanos needs to kill Nick Fury. It'll mean that a really familiar element is now gone, something Joss Wheadon loves to take away. More than that, the organization of Shield would collapsed upon itself, which would have a huge impact on the MCU. The Avengers would need to reorganize themselves into their own group, and that...will cause **** to go down.